Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, shall mean not only materials containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also materials which contain suitable replacement atoms for such silicon and aluminum, such as gallosilicates, silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, shall mean zeolite materials consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
Synthetic zeolites are normally prepared by the crystallization of zeolites from a supersaturated synthesis mixture. The resulting crystalline product is then dried and calcined to produce a zeolite powder. Although the zeolite powder has good adsorptive properties, its practical applications are severely limited because it is difficult to operate fixed beds with zeolite powder. Therefore, prior to using the powder in commercial processes, the zeolite crystals are usually bound.
The zeolite powder is typically bound by forming a zeolite aggregate such as a pill, sphere, or extrudate. The extrudate is usually formed by extruding the zeolite in the presence of a non-zeolitic binder and drying and calcining the resulting extrudate. The binder materials used are resistant to the temperatures and other conditions, e.g., mechanical attrition, which occur in various hydrocarbon conversion processes. Examples of binder materials include amorphous materials such as alumina, silica, titania, and various types of clays. It is generally necessary that the zeolite be resistant to mechanical attrition, that is, the formation of fines, which are small particles, e.g., particles having a size of less than 20 microns.
Although such bound zeolite aggregates have much better mechanical strength than the zeolite powder, when such a bound zeolite is used for hydrocarbon conversion, the performance of the zeolite catalyst, e.g., activity, selectivity, activity maintenance, or combinations thereof, can be reduced because of the binder. For instance, since the binder is typically present in an amount of up to about 50 wt. % of zeolite, the binder dilutes the adsorption properties of the zeolite aggregate. In addition, since the bound zeolite is prepared by extruding or otherwise forming the zeolite with the binder and subsequently drying and calcining the extrudate, the amorphous binder can penetrate the pores of the zeolite or otherwise block access to the pores of the zeolite, or slow the rate of mass transfer to the pores of the zeolite which can reduce the effectiveness of the zeolite when used in hydrocarbon conversion. Furthermore, when the bound zeolite is used in hydrocarbon conversion, the binder may affect the chemical reactions that are taking place within the zeolite and also may itself catalyze undesirable reactions, which can result in the formation of undesirable products.
For certain hydrocarbon conversion processes, it is sometimes desirable that the zeolite catalysts be tailored to maximize their performance. One method for tailoring zeolite catalysts is to bind zeolite core crystals with binder crystals of a zeolite having a structure type that is different from the core crystals. Such catalysts are disclosed in PCT Publication WO PCT/US97/45198.
Zeolite catalysts comprising zeolite core crystals which are bound together by binder crystals of a zeolite having a different structure type different from the core crystals can be bifunctional, i.e., capable of performing two or more functions. For example, the catalysts can induce separate reactions (the zeolite core crystals and zeolite binder crystals each inducing reactions). Also, the zeolite binder crystals can reduce the amount of reactions taking place on the surface of the zeolite core crystals. Still further, the zeolite binder crystals can reduce accessibility of reactants to the surface of the zeolite core crystals by selectively sieving molecules in the hydrocarbon feedstream based on their size or shape to prevent undesirable molecules present in the feedstream from entering the catalytic phase of the zeolite core crystals and/or selectively sieve desired molecules based on their size or shape in order to prevent undesirable molecules from exiting the catalyst phase of the core crystals.
One procedure for making zeolite-bound zeolite involves converting the silica present in the silica binder of a silica-bound zeolite aggregate to a zeolite binder. The procedure involves aging the silica bound aggregate for sufficient time in an aqueous alkaline solution. When such a procedure is used to produce MFI-bound zeolite having zeolite core crystals with structure type other than MFI, certain problems can arise which result in the MFI-bound zeolite having less than desirable strength and integrity. For instance, if the alkalinity of the aging solution is too high or the time needed for conversion of the silica binder is too long, the core crystals and/or silica-bound aggregate can lose their integrity. This can result in the MFI-bound zeolite having less than acceptable strength and/or integrity.
The present invention provides a process for preparing zeolite (other than MFI) core crystals that are bound by MFI structure type zeolite which overcomes or at least mitigates the above described problems. The zeolite bound zeolite made by the process finds particular application as an adsorbent or as a catalyst in hydrocarbon conversion.